1. Field of the Invention
This invention relates generally to the field of manhole construction. In particular, it relates to a seal which externally seals an assembly against surface water infiltration between the manhole casting and portions of the supporting structure and through the supporting structure itself.
2. Description of the Prior art
In a conventional manhole assembly a manhole casting, which constitutes the uppermost portion of the assembly and serves as the seat of a manhole cover, rests on or is fastened to a supporting structure. This supporting structure may be a manhole cone or there may be one or more adjusting rings between the cone and the casting. If the adjusting rings are used the portion of structure intermediate the casting and the cone is called the manhole chimney. The interface between the casting and the cone or adjusting ring on which it rests consists generally of two opposing flat surfaces.
In the past manhole chimneys have been constructed with precast adjusting rings, or brick or block, and have been used on manhole cones constructed with precast sections, brick or block, or cast in place concrete. Existing manholes may have had the casting shimmed with wood or bricks, with mortar placed in the gaps between shims.
One problem with conventional manhole assemblies is that surface water can infiltrate the manhole at the interfaces between the casting and the cone or between the casting and the adjusting rings, or between the adjusting rings.
As the manhole assembly ages the problem of water infiltration becomes even greater, because of deterioration in the supporting structure.
One cause of deterioration is the freeze/thaw cycle common to much of the United States during winter and spring. This causes a breakdown in the interfaces in the manhole assembly. Surface water easily infiltrates between such deteriorated interfaces.
Another cause of deterioration from prolonged use is the weight of traffic passing over the manhole. Manhole assemblies are commonly located beneath the surface of a road, with the manhole cover and top portion of the casting being flush with the road surface. The weight of the vehicles passing over the assembly commonly causes interfaces to deteriorate, and also creates cracks in the road surface surrounding the manhole assembly. Surface water runs through these cracks and infiltrates the deteriorated structure.
These same factors can also cause a vertical or horizontal displacement of the casting relative to the supporting structure, which further increases the probability of water infiltration.
Water infiltration into the sewage collection system represents a major problem in sewage treatment. The capacity of a sewage treatment system in large part is a measure of the volume of the effluent it can treat. Water infiltration during rain storms or during periods of extended rainfall activity adds to the total volume of effluent treated. This increased volume of flow may overload new and old sewage treatment systems. In most cases, the excess volume of the effluent overload is dumped untreated into rivers or lakes. It is believed that water infiltration through manhole assemblies is a primary contribution to the overloading of sewage treatment systems.
Another problem which results from surface water infiltration of manhole assemblies is the broad dissemination of contaminated surface water, especially when the contaminant is a petro chemical or dangerous man-made pollutant. Contaminated surface water which infiltrates the sewage system through a manhole will be distributed to other sites by the sewage lines or water runoff lines to which the manhole assemblies are connected. Thus, a contaminant that should be contained and disposed of safely away from population centers is, instead, widely dispersed in an uncontrolled fashion.
Consequently there is a continuing need in the field of manhole construction for an apparatus to seal the assembly against surface water infiltration. There is also a need for a seal to be effective against infiltration occurring in the area between the casting and the supporting structure and through the supporting structure. There is also a need for a seal that can accommodate vertical and horizontal displacement of the casting relative to the supporting structure during prolonged use. There is also a need for the seal to be economically manufactured and simply constructed so that it may be easily applied in the field. There is also a need for a seal that does not interfere with the normal use of the manhole.
In the prior art most inventive activity has been directed to sealing pipe sections together and to sealing ingoing and outgoing pipe at the places where it enters and leaves the lower levels of the manhole assembly. Preventing leakage at those points does not solve the problem of surface water infiltration through the supporting structure or the casting interface.
Three prior patents have been issued which relate to improvements to the placement of manhole castings.
U.S. Pat. No. 3,308,727 provides a cushion for the manhole cover but does not seal it.
U.S. Pat. No. 4,029,425 provides an extension for a manhole. A bicycle inner tube encircles the extension where the extension fits into the top of the manhole. The innertube can be inflated after the extension is in place to fill a channel between the extension and the side of the manhole. This should prevent some infiltration, but would only do so at the level of the bicycle innertube. It will not prevent infiltration at any other level in the supporting structure of the manhole.
U.S. Pat. No. 4,305,679 shows an internal sealing device to prevent water entering the joint between the manhole casing and the manhole. The seal is held in place inside the manhole assembly by two brace arrangements, each consisting of an outer expandable rim which presses the seal against the inside surface of the manhole assembly and is joined to an inner hub by threaded spokes. The brace arrangements block entrance to and exit from the manhole, thus entirely eliminating the primary function of the manhole. That is, they prevent normal use of the manhole when the seal is in place. In a second embodiment the manhole is completely sealed by a continuous membrane stretched across it.
Applicants' invention disclosed in copending U.S. patent application No. 340,232 also addresses the problems present in the field and eliminates them by providing an internal seal for a manhole assembly, which can seal the assembly against water infiltration through the interfaces between manhole casting and supporting structure while allowing for horizontal and vertical displacement of the casting relative to the supporting structure, without interfering with normal ingress to and egress from the manhole assembly. However, some uses of the manhole assembly may present a high risk of damage to an internal seal. For example, if equipment is to be routinely or repeatedly moved through the assembly the seal is more likely to be damaged. Under those circumstances an external seal is desirable. It may also be more convenient to seal externally, such as during new construction.